The surface acoustic wave (SAW) sensor offers advantages in that it is wireless, passive, small and has varying embodiments for different sensor applications. Surface acoustic wave (SAW) sensors are capable of measuring physical, chemical and biological variables and have the ability to operate in harsh environments. In addition, there are a variety of ways of encoding the sensed data information for retrieval. Single sensor systems can typically use a single carrier RF frequency and a simple device embodiment, since tagging is not required. In a multi-sensor environment, it is necessary to both identify the sensor as well as obtain the sensed information. The SAW sensor then becomes both a sensor and a tag and must transmit identification and sensor information simultaneously.
Known SAW devices include delay line and resonator-based oscillators, differential delay lines, and devices utilizing multiple reflective structures. Single sensor systems can typically use a single carrier frequency and a simple coding technique, since tagging is not required. However, there are advantages of using spread spectrum techniques for device interrogation and coding, such as enhanced processing gain and greater interrogation power.
The use of orthogonal frequencies for a wealth of communication and signal processing applications is well known to those skilled in the art. Orthogonal frequencies are often used in an M-ary frequency shift keying (FSK) system. There is a required relationship between the local, or basis set, frequencies and their bandwidths which meets the orthogonality condition. If adjacent time chips have contiguous local stepped frequencies, then a stepped chirp response is obtained. See S. E. Carter and D. C. Malocha, “SAW device implementation of a weighted stepped chirp code signal for direct sequence spread spectrum communication systems”, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency control, Vol. 47, July 2000, pp. 967-973.
Co-pending U.S. patent application Ser. No. 11/508,674 filed on Aug. 23, 2006 assigned to the same assignee as the present application, teaches weighted surface acoustic wave reflector gratings for coding identification tags and sensors to enable unique sensor operation and identification for a multi-sensor environment. In an embodiment, the weighted reflectors are variable while in another embodiment the reflector gratings are apodized. The weighting technique allows the designer to decrease reflectively and allows for more chips to be implemented in a device and, consequently, more coding diversity. As a result, more tags and sensors can be implemented using a given bandwidth when compared with uniform reflectors. Use of weighted reflector gratings with OFC makes various phase shifting schemes possible, such as in-phase and quadrature implementations of coded waveforms resulting in reduced device size and increased coding. The device may include a single transducer/antenna pair with a bank of reflectors on one side of the transducer/antenna pair, or a bank of reflectors on both sides of the transducer/antenna pair, or alternatively, a unidirectional transducer may be used to reduce the device loss and size.